Today is Earth Day, the internationally designated time to pay extra-special attention to human impact on Earth’s ecology. But in addition to your Earth Day observances, why not join NASA scientists this year in celebrating Sun-Earth Day, an astronomically educational holiday designed to recognize special moments between Earth and our Sun?

Past Sun-Earth Days have celebrated everything from Sun watching in culture (like ancient Egyptian pillars to the sun god Re and the Wijiji Great House in Chaco Canyon, New Mexico) to space weather, magnetic storms, and eclipses. In accordance with NASA’s annual declaration, Sun-Earth Day falls on a different day every year, coincident with some special solar event between the spring equinox and summer solstice.

The Transit of Venus

NASA announced the first Sun-Earth Day in 2004, to celebrate the Transit of Venus – a rare celestial event in which Venus passes between Earth and the Sun, briefly casting its shadow over the latter. The event is different than a solar eclipse, which—caused by the moon passing between Earth and the Sun—blocks the Sun from our earthling view. Because Venus is much farther away than the moon, its shadow appears as a tiny dot moving across the solar disk.

Before June 8th/Sun-Earth Day 2004, the Venus Transit hadn’t occurred since 1882. The event caused a huge stir in the U.S. at the time—John Phillip Sousa even composed a march for the occasion. The hype began with Venus’ previous transit of that cycle, in 1874, when Hawaiian King David Kalakaua invited astronomers from all over the world to view the event on Hawaii, where the unobstructed 360-degree equatorial view provided one of the best vantage points on Earth. This historic transit allowed American astronomer Simon Newcomb, building upon hundreds of years of astronomical research, to triangulate the approximate distance between the Earth and the sun, a measurement known as an Astronomical Unit.

This year, the Transit of Venus is happening again: June 6th, 2012, at around sunset Eastern Time. Make sure to mark your calendars and get your safe viewing gear in order, because Venus won’t transit again until 2117. Best Sun-Earth Day ever!